1. Field of the Invention
A method for controlling a heating plant adapted for local heating of a surface to avoid ice and snow on this surface, in such a manner that a low energy consumption is obtained. The invention also relates to a control system for such a heating plant.
The invention especially relates to controlling of a heating plant in such a manner that surfaces provided with heating elements heated by energy supplied from e.g. electrical energy sources or from a heated fluid are considered. The invention is substantially, but not exclusively used for local heating of areas with traffic such as streets or sidewalks or selected parts of such areas.
In the following specification a heating plant adapted to heat a part of a street by means of electrical energy substantially will be described, however, this is only an example as the invention also relates to plants heating other objects, such as points for traffic on tracks or rails and heating plants using other forms of energy than electricity, e.g. heating flows of fluids or combinations of such plants.
2. Description of the Background Art
Previously, many kinds of street heating plants are known adopted for the melting of snow. The simplest plants are built as an electric heating loop provided below the surface where an area without snow and ice is desired. Such a plant is, in its most simple embodiment, made without any control system as it is provided only with an xe2x80x9conxe2x80x9d/xe2x80x9coffxe2x80x9d switch which may be operated manually, and then the plant is switched xe2x80x9conxe2x80x9d during cold periods when it is desirable that snow and ice shall melt and flow away, while the plant is switched xe2x80x9coffxe2x80x9d during warmer periods or when there is no precipitation in the shape of snow. Accordingly such plants have to be supervised constantly by a person present on the site and is technical spoken without any real control system.
The above mentioned simple and manually controlled plants will necessarily result in high operating and energy costs, both as they require constant monitoring and as the users sometimes will forget to switch the plant xe2x80x9coffxe2x80x9d when required. Accordingly, automatic plants have been developed provided with different kinds of detectors controlling the switching xe2x80x9conxe2x80x9d and switching xe2x80x9coffxe2x80x9d operations. Here only one plant should be mentioned, described in Norwegian patent application No. 92.3375 filed by the same applicant, and here an automatic plant is shown using a temperature detector having two thermostats or relays, and also a humidity detector in the surface of a street. This detector will, together with the relays and electronic circuits adapted therefore, supply energy to the heating element only when precipitation and a surface temperature below zero occurs simultaneously. Then the plant will be switched xe2x80x9conxe2x80x9d only when precipitation takes place in periods with a temperature below zero and be switched xe2x80x9coffxe2x80x9d both when the temperature rise above a predetermined lower threshold value and also when no precipitation is detected (no humidity detected).
Even when plants of the latter type is considered the energy consumption often will be higher than required. This depends especially on two conditions:
1) The ground or surface temperature ought to be maintained on a rather high xe2x80x9cpreparedness levelxe2x80x9d during cold and dry periods. Otherwise the surface would, without any kind of energy supply during such periods, be extremely cooled down, and if snow should start falling after such a long lasting cold period it would take a very long time before the surface is sufficiently heated from its extremely low temperature so that melting of snow will start. Accordingly, long periods with slush would occur in the areal. By means of empiric experiments it has been found that such a xe2x80x9cpreparedness heatingxe2x80x9d of the surface during long lasting cold periods in many places will represent the highest portion of the energy consumption of such plants; in the area of the capital Oslo in Norway about 80%.
2) The users of such plants will often adjust the stand-by temperature upwards more than required to avoid that snow will build up in the first time of each new fall of snow. Such a xe2x80x9cfaulty settingxe2x80x9d of the plant will give unnecessarily high working costs.
Another disadvantage with plants comprising heat detectors in the ground is that such detectors easily may be damaged by heavy traffic loads and therefore deactivated whereafter the control of the plant will be delusive. With a control system according to the present invention it will not be necessary to use temperature detectors to measure the earth temperature. Although existing plants comprising a detector for detecting the ground temperature may easily be re-designed to act according to the present invention, so that the thermostat for earth temperature may be removed or deactivated as its coupling temperature is adjusted below the existing temperature level. (In addition one or more relatively inexpensive components may be added to the circuit during such a re-designing process, as closer explained below).
Practical examinations have shown that the power consumption of a street heating plant may be reduced to less than the half by undertaking the control process according to the present invention. The economical advantages of such a solution will of course depend of the power price, but normally the additional costs of such a plant will be so low that the plant costs according to the invention will be saved after a short operating period.
The object of the present invention is to provide a method for optimal control of a heating plant in such a manner that the power consumption is as low as possible without giving long periods with snow, ice or slush on the site. The object also is to provide a control system which does not imply the disadvantages mentioned above, but which to the contrary results in a cost effective working of the heating plant without any local maintenance. The power consumption then may be reduced from approximately 400 kWh/m2/year to about 150 kWh/m2/year (an example valid for street heating plants in the Oslo-region in south Norway), without other changes than explained above.
In very cold regions having relatively small amounts of precipitation during the cold periods, much of the power consumption has so far been used to keep a high stand-by temperature to avoid troublesome amounts of slush, which will be the result if the plant is not switched xe2x80x9conxe2x80x9d before a snowfall begins.
By using a method and a control system according to the below claims, these advantages may be obtained. As a principle it may be said that the invention is based on good and correct forecasts relating to anticipated temperatures and anticipated precipitation amounts in form of snow. With good forecasts a plant may be switched xe2x80x9coffxe2x80x9d during long lasting cold periods without precipitation, no matter how low the temperature is. Only when precipitation having the shape of snow has been included in the weather forecast it is required to switch the plant xe2x80x9conxe2x80x9d and then at a determined period before the fall of snow starts. This time period may again be a function of the temperature at the moment, as the plant should be switched xe2x80x9conxe2x80x9d well in time before the fall of snow starts, especially when the temperature is very low, to secure that a suitable start temperature is obtained before the beginning of the snowfall. If the temperature is close to 0xc2x0 C. and has been at this level for a long period, the switching xe2x80x9conxe2x80x9d of the plant may wait until the very beginning of the snowfall. In such a manner the duration of the time period from switching xe2x80x9conxe2x80x9d of the plant until the snow begins to fall, may be determined dependent of the preceding temperature changes. A thermostat monitoring the earth temperature may then also be superfluous in the plant, which also will lead to reduction of the working costs.
All the above advantages and objects may be met by using a method or a control system according to the patent claims stated below.